Influenza, commonly known as "the flu," is a highly contagious viral infection of the respiratory tract. It affects all age groups, though kids tend to get it more often than adults.

In the United States, flu season runs from October to May, with most cases happening between late December and early March.

Signs and Symptoms

The flu is often confused with the common cold, but flu symptoms usually are more severe than the typical sneezing and stuffiness of a cold.

Symptoms, which usually begin about 2 days after exposure to the virus, can include:

fever

chills

headache

muscle aches

dizziness

loss of appetite

tiredness

cough

sore throat

runny nose

nausea or vomiting

weakness

ear pain

diarrhea

Infants with the flu also may seem fussy all of a sudden or just "not look right."

Duration

After 5 days, fever and other symptoms have usually disappeared, but a cough and weakness may continue.

All symptoms are usually gone within a week or two. However, it's important to treat the flu seriously because it can lead to pneumonia and other life-threatening complications, particularly in babies, senior citizens, and people with long-term health problems.

Contagiousness

The flu is contagious, spread by virus-infected droplets that are coughed or sneezed into the air. People who are infected are contagious from a day before they feel sick until their symptoms have ended (about 1 week for adults, but this can be longer for young kids).

The flu usually happens in small outbreaks, but epidemics — when the illness spreads rapidly and affects many people in an area at the same time — tend to occur every few years. Epidemics often peak within 2 or 3 weeks after the first cases are reported.

When an epidemic spreads worldwide, it's called a pandemic. The most recent flu pandemic was in 2009-2010, with the H1N1 ("swine") flu.

The Flu Vaccine

Routine annual influenza vaccination is recommended for everyone 6 months of age and older. It's usually offered between September and mid-November, but may be given at other times of the year.

The vaccine helps protect people from the flu viruses that experts think will be most common in the upcoming flu season. While the vaccine doesn't completely guarantee against getting sick, someone who's been vaccinated and still gets the flu will have fewer and milder symptoms.

Flu vaccines are available as a shot or nasal mist:

Given as an injection, the flu shot contains killed flu viruses that will not cause the flu, but will prepare the body to fight off infection from that particular type of live flu virus if someone comes into contact with it.

The nasal mist contains weakened live flu viruses that cannot cause the severe symptoms typically associated with the flu, but can cause some mild symptoms. So people with weakened immune systems or certain health conditions should not get the nasal mist vaccine. It is only for healthy, non-pregnant people between 2 and 49 years old, and is preferred over the flu shot for kids ages 2 to 8.

When & Where to Get It

People who got the vaccine one year aren't protected from getting the flu the next because the protection wears off and flu viruses constantly change. That's why the vaccine is updated each year to include the most current strains of the virus.

So to have the best protection against the flu, it's important to get the vaccine every year.

The seasonal flu vaccine becomes available each fall. It is given in places like hospitals, clinics, community centers, pharmacies, doctor's offices, and schools. Many kids receive the flu vaccine at school.

Kids younger than 9 years old who are getting the flu vaccine for the first time will receive two separate doses at least a month apart. Those under 9 who have received the flu vaccine before still might need two doses if they did not receive at least two vaccines since July 2010, or if the number of shots they’ve received since July 2010 is unknown. This is to ensure that all children are vaccinated against the H1N1 flu strain that surfaced in 2009. Kids older than 9 years old only need one dose of the vaccine.

It can take about 2 weeks after the vaccine for the body to build up protection to the flu. Getting the vaccine before the flu season is in full force gives the body a chance to build up immunity to (protection from) the virus.

You can get a flu vaccine well into flu season, but it's best to try to get it as early as possible so the body has time to build up immunity. However, even in January there are still a few months left in the flu season, so it's still a good idea to get protection.

If you are traveling outside the country, be sure to check with your doctor because the flu season may be different in other countries. If you are traveling to a place with flu activity, make sure you are vaccinated at least 2 weeks before travel.

Possible Side Effects

While very few people get side effects from the flu shot, those who do may have soreness or swelling at the site of the injection or mild side effects, such as headache or low-grade fever.

Some people who get the nasal spray vaccine may develop mild flu-like symptoms, including runny nose, headache, vomiting, muscle aches, and fever.

Although these side effects might last for a day or so, flu viruses can potentially sicken someone for weeks and cause health problems that could require hospitalization, especially in young children or people with chronic diseases. So doctors believe that the benefits of getting the flu vaccine outweigh any potential risks.

Who Is Considered High Risk?

Although flu vaccine is recommended for everyone aged 6 months and older, it is especially important for people at higher risk of health problems from the flu to get vaccinated. They include:

all kids 6 months through 4 years old (babies younger than 6 months are also considered high risk, but they cannot receive the flu vaccine)

anyone 65 years and older

all women who are pregnant, are considering pregnancy, have recently given birth, or are breastfeeding during flu season

anyone whose immune system is weakened from medications or illnesses (like HIV infection)

residents of long-term care facilities, such as nursing homes

any adult or child with chronic medical conditions, such as asthma

kids or teens who take aspirin regularly and are at risk for developing Reye syndrome if they get the flu

caregivers or household contacts of anyone in a high-risk group (like children younger than 5 years old, especially those younger than 6 months, and those with high-risk conditions)

Native Americans and Alaskan natives

Certain things might prevent a person from getting the vaccine. Talk to your doctor to see if the vaccine is still recommended if your child:

has ever had a severe reaction to a flu vaccination

has Guillain-Barré syndrome (a rare condition that affects the immune system and nerves)

In the past, it was recommended that anyone with an egg allergy talk to a doctor about whether receiving the flu vaccine was safe because it is grown inside eggs. But health experts now say that the amount of egg allergen in the vaccine is so tiny that it (but not the nasal mist) is safe even for kids with a severe egg allergy. This is especially important during a severe flu season.

Still, a child with an egg allergy should get the flu shot in a doctor's office, not at a supermarket, drugstore, or other venue. And if the allergy is severe, it might need to be given in an allergist's office.

If your child is sick and has a fever, talk to your doctor about rescheduling the flu shot.

Preventing the Flu From Spreading

There's no guaranteed way — including being vaccinated — to have 100% protection from the flu. But these practical steps make spreading the flu less likely:

Wash your hands well and often with soap, especially after using the bathroom, after coughing or sneezing, and before eating or preparing food.

Never pick up used tissues.

Don't share cups and eating utensils.

Stay home from work or school when you're sick with the flu.

Cover your mouth and nose with a tissue when you cough or sneeze, then put it in the trash. If a tissue isn't available, cough or sneeze into your upper arm, not into your hands.

Treatment

Cases of the flu rarely require specific medical treatment. But some kids with chronic medical conditions (such as asthma, diabetes, and HIV) or children under 2 years old might become sicker with the flu and may have a greater risk of complications. Some kids with the flu need to be hospitalized.

For a severely ill child or one with other special circumstances, a doctor may prescribe an antiviral medicine that can decrease the duration of illness by 1-2 days and prevent potential complications of the flu. This medicine is only helpful if given within 48 hours of the onset of the flu. Most healthy people who get the flu do not need to take an antiviral medication. If an antiviral medication is prescribed, be sure to discuss any possible side effects with your doctor.

These at-home tips can help most otherwise healthy kids cope with the flu. Have them:

drink lots of fluids to prevent dehydration

get plenty of sleep and take it easy

take acetaminophen or ibuprofen to relieve fever and aches (do not give aspirin to children or teens as it may cause a rare but serious illness called Reye syndrome)

wear layers, since the flu often makes them cold one minute and hot the next (wearing layers — like a T-shirt, sweatshirt, and robe — makes it easy to add or subtract clothes as needed)

Kids who are sick should stay home from school and childcare until they are without fever for at least 24 hours without the use of a fever-reducing medicine. Some might need to stay home longer, depending on how they feel. If you have questions or concerns, talk to your doctor.

When to Call the Doctor

Call the doctor if your child:

has flu symptoms

has a high fever, or fever with a rash

has trouble breathing or rapid breathing

has bluish skin color

is not drinking enough fluids

seems very sleepy or lethargic

seems confused

has flu symptoms that get better, but then get worse

For the most part, though, the flu is usually gone in a week or two with a little rest and tender loving care.